GB 2235287 discloses an optical sensor based on the use of surface plasmon polaritons (SPP). The sensor comprises apparatus for detecting an SPP resonance maximum which occurs following polarisation conversion of particular wavelengths of radiation incident upon a surface which correspond to the excitation of an SPP at or about its resonant frequency.
WO 98/37514, the contents of which are hereby incorporated by reference in their entirety, also makes use of the polarisation conversion effect. In WO 98/37514, a signature recognition system is disclosed comprising one or more suitably profiled diffraction gratings provided on an article, a source of polarised radiation, means for directing the source of polarised radiation onto the grating(s) at a suitable plane of incidence and means for detecting reflected radiation which is oppositely polarised to the incident radiation. WO 98/37514 discloses that the system can be used to distinguish effects at different wavelengths and/or provide identification codes such as bar codes. Bar code systems are well known as a means of distinguishing articles, but can easily be distorted by creases, scratches and so on. This can cause errors in the detection step.
There is a continued need for alternative and/or improved methods of marking and/or authenticating articles. In particular, it would be desirable to be able to provide articles which have visually appealing and/or hidden features, yet are tolerant to damage in everyday use.